In the field of firearms, ammunition cartridges (also called ammunition shells and “rounds”) contain powder, which when ignited propels a bullet down the barrel of a gun toward a target. Prior art cartridges, particularly those for use with small arms, typically comprise a casing made of forged brass. A propellant, typically smokeless explosive powder, is contained within the casing and is ignited by impact of a firing pin of the breech block of the gun on a primer that is set in a recess at the base of the casing.
A cartridge relevant to the present invention comprises a generally cylindrical casing having a substantially closed end, called here the base end, and an opposing open end, often called the mouth. The open end of the mouth receives a bullet after propellant is put into the concavity of the casing; and the casing is crimped as needed around the bullet to hold it in place. The base end of the casing typically has a cannelure or groove to enable the casing to be engaged by grips on the firing mechanism of a gun. The base end often comprises a larger diameter flange portion which acts as a stop, limiting the depth of insertion of the cartridge into the chamber of the barrel of the gun.
In prior art cartridges comprised of forged metal (typically cartridge brass) casings, the base end which holds the primer is integral with the sleeve portion which holds the bullet. Typically, the sleeve is tapered internally (with the larger diameter at the open end), attributable to the metalworking process by which the cartridge is formed. The wall thickness near the base may is as several times the thickness of the wall at the mouth end. Sometimes a casing has a step-down in diameter in vicinity of the open end, where the bullet is captured.
The exterior surface of the base of the typical cartridge has a recess within which is contained a percussion primer that contains a small quantity of impact-sensitive explosive powder. Typically the primer is in the center of the base and comprises an internal anvil which is supported during the firing process by the end of the recess. There is a small passageway through the base, often called the vent or flash hole, enabling ignited primer gases to pass through the base and into the concavity of the casing, to ignite the propellant.
A cartridge necessarily slip fits into a chamber of the barrel of the gun for which it is intended. A cartridge is typically inserted and held in place by the breech block (also called slide or bolt) which usually has one or more claws for grasping the groove in the rim of the base of the casing. When the primer is struck by the firing pin within the breech block of the gun, the propellant explosively turns into gas and forces the bullet from the cartridge and down the bore of the barrel of the gun. In that process, the pressure of the deflagrated propellant gas expands the casing of the cartridge radially outwardly, desirably creating a seal against gas escape through the slip fit clearance region of the casing with the chamber. Then the casing hopefully relaxes, moving radially inwardly to about its original dimension, enabling the casing of the spent cartridge to be readily removed. A casing is often removed from the chamber by retracting action of the breech block which pulls on the cannelure; or by force of the pressurized gases on the casing in coordination with rearward motion of the breech block. After ejection from the breech area, a casing may often be recycled by replacing the primer and powder and installing a new bullet.
There are differences in peak deflagration pressures that occur during firing of a cartridge, according the size of the cartridge and the volume of gunpowder it holds. Peak pressures, e.g., 20,000-30,000 pounds per square inch (psi), are typically encountered in cartridges firing bullets in hand guns. A 9 mm pistol cartridge is an example. Comparatively high pressures, for e.g., 40,000-60,000 psi can be encountered in cartridges firing bullets in rifles, particularly military rifles. A 7.62 mm rifle cartridge is an example.
Good cartridges have a number of desired characteristics, with the ranking determined by a particular customer and application. Generally stated, cartridges and casings desirably have the following features. They should be strong enough to resist fracture or distortion which causes jamming, when subjected to the pressure of deflagration gases as just discussed. They should be configured for making a seal with the chamber of the gun during firing of a bullet; and they should be readily extractable from the chamber. They should have general durability and integrity, including the ability to resist possible rough handling prior to placement in a firearm and the ability to be reworked and reloaded. They should be corrosion resistant. Traditionally, cartridges of brass alloys had worked well. Further, particularly for military applications there is a desire for a casing that is as light as possible particularly when quantities of cartridges are carried by an aircraft or by a soldier.
The brass of common and widely used traditional casings is a costly alloy compared to various iron and aluminum alloys, and of course, compared to plastic. However, alternative materials such as steels, aluminum alloys and plastics have found less favor in the marketplace, usually due to perceived deficiencies in the characteristics above.
Other inventors have described a variety of alternative constructions and materials for cartridges. For example: Cartridges may be made in whole or part from plastics and metals other than brass. The casing may be made of plastic or paper and attached to a metal base (as is common for shotgun shells). The casing may be made in the form of a sleeve having a nipple end which is inserted into a passageway in a base that runs to a primer, and the nipple is flared radially, to hold the two parts together. For reference see the following publications: Milbank U.S. Pat. No. 125,830; Horn U.S. Pat. No. 3,688,699; Skochko U.S. Pat. No. 3,765,297; Anderson U.S. Pat. No. 3,977,326; Horn U.S. Pat. No. 3,688,699; Dittrich U.S. Patent Publications 2007/0214992 and 2008091245, and Neugebauer U.S. Patent Publication 2014/0224144. Based on the absence from the marketplace for most if not all of the foregoing kinds of casings and cartridges, it would appear further improvements are needed.
Further desirable characteristics of casings include the following: For economic and environmental reasons, spent casings that are expelled from a gun after firing will be amenable to being readily gathered up; when casings are made of brass or plastic that basically means using visual or optical means. Also for economic and environmental reasons, a casing will be able to be re-used; but the construction of many prior art casings does not lend itself to one or more re-uses. There is always a desire for a lower cost cartridge, particularly for small arms cartridges that are used in large quantities. And there is always a continuing desire to improve the performance of cartridges. For example, for any particular caliber of cartridge it is generally desirable to maximize the volume within which gunpowder is contained.